Trunked radio repeater system

ABSTRACT

A digitally trunked radio repeater system provides substantial improvements in timeliness of channel acquisition and channel drop, and in reliability of critical control signalling. The system uses a much higher digital signalling rate than is typically found in prior art systems, and uses a control channel to convey digital channel request and assignment messages between the central site and mobile transceivers. The mobile radio transceivers transmit channel requests on the control channel (if no response is received, the mobile retries during a retry time window which increases in duration in dependence on the number of retries). The mobile transceiver switches to a working channel in response to an assignment message received on the control channel. Subaudible digital signals transmitted on the control channel and on active working channels allow late entry, shifting to higher priority calls, and other advanced functions. Message and transmission trunking capabilities are both present so as to maximize working channel usage without compromising channel access for high priority communications. During transmission trunking, called and calling receivers return to the control channel after each transmission (and called transceivers may be inhibited from transmitting) but grant higher priority to calls from the other transceivers being communicated with to ensure continuity over an entire conversation. Additional functions and fault tolerant features further increase the versatility and reliability of the system.

This is a divisional of application Ser. No. 08/105,153, filed Aug. 12, 1993, now U.S. Pat. No. 5,483,670 issued Jan. 9, 1996, which in turn is a divisional of Ser. No. 07/860,159 filed Mar. 30, 1992, now U.S. Pat. No. 5,274,837 issued Dec. 28, 1993, which in turn is a divisional of Ser. No. 07/464,053 filed Jan. 3, 1990, now U.S. Pat. No. 5,125,102 issued Jun. 23, 1992 which in turn is a divisional of application Ser. No. 07/056,922 filed Jun. 3, 1987, now U.S. Pat. No. 4,905,302, issued Feb. 27, 1990.

BACKGROUND AND SUMMARY OF THE INVENTION

This invention is generally directed to the art of trunked radio repeater systems. It is more particularly directed to such systems using digital control signals transmitted over a dedicated control channel while also using plural working channels which are assigned temporarily for use by individual radio units.

The trunking of radio repeaters is well-known. Early trunking systems used analog control signals while some more recent systems have utilized digital control signals. Control signals have been utilized on a dedicated control channel and/or on different ones of the working channels for various different reasons and effects. A non-exhaustive but somewhat representative sampling of prior art publications and patents describing typical prior art trunked radio repeater systems is identified below:

U.S. Pat. No. 3,292,178--Magnuski (1966)

U.S. Pat. No. 3,458,664--Adlhoch et al (1969)

U.S. Pat. No. 3,571,519--Tsimbidis (1971)

U.S. Pat. No. 3,696,210--Peterson et al (1972)

U.S. Pat. No. 3,906,166--Cooper et al (1975)

U.S. Pat. No. 3,936,616--DiGianfilippo (1976)

U.S. Pat. No. 3,970,801--Ross et al (1976)

U.S. Pat. No. 4,001,693--Stackhouse etal (1977)

U.S. Pat. No. 4,010,327--Kobrinetz et al (1977)

U.S. Pat. No. 4,012,597--Lynk, Jr. et al (1977)

U.S. Pat. No. 4,022,973--Stackhouse etal (1977)

U.S. Pat. No. 4,027,243--Stackhouse etal (1977)

U.S. Pat. No. 4,029,901--Campbell (1977)

U.S. Pat. No. 4,128,740--Graziano (1978)

U.S. Pat. No. 4,131,849--Freeburg et al (1978)

U.S. Pat. No. 4,184,118--Cannalte et al (1980)

U.S. Pat. No. 4,231,114--Dolikian (1980)

U.S. Pat. No. 4,309,772--Kloker et al (1982)

U.S. Pat. No. 4,312,070--Coombes et al (1982)

U. S. Pat. No. 4, 3 12,074--Pautler et al (1982)

U.S. Pat. No. 4,326,264--Cohen et al (1982)

U.S. Pat. No. 4,339,823--Predina et al (1982)

U.S. Pat. No. 4,347,625--Williams (1982)

U.S. Pat. No. 4, 3 60,927--Bowen et al (1982)

U.S. Pat. No. 4,400,585--Kamen et al (1982)

U. S. Pat. No. 4,409,687--Berti et al (1983)

U.S. Pat. No. 4,430,742--Milleker et al (1984)

U.S. Pat. No. 4,430,755--Nadir et al (1984)

U.S. Pat. No. 4,433,256--Dolikian (1984)

U.S. Pat. No. 4,450,573--Noble (1984)

U.S. Pat. No. 4,485,486--Webb et al (1984)

U.S. Pat. No. 4,578,815--Persinotti (1985)

Bowen et al is one example of prior art switched channel repeater systems which avoid using a dedicated control channel--in part by providing a handshake with the repeater site controller on a seized "idle" working channel before communication with the called unit(s) is permitted to proceed.

There are many actual and potential applications for trunked radio repeater systems. However, one of the more important applications is for public service trunked (PST) systems. For example, one metropolitan area may advantageously utilize a single system of trunked radio repeaters to provide efficient radio communications between individual radio units within many different agencies. Each agency may, in turn, achieve efficient communication between individual units of different fleets or sub-units (e.g., the police department may have a need to provide efficient communications between different units of its squad car force, different portable units assigned to foot patrolmen, different units of detectives or narcotics agents and the like). Sometimes it may be important to communicate simultaneously to predefined groups of units (e.g., all units, all the squad cars, all of the foot patrolmen, etc.). At the same time, other agencies (e.g., the fire department, the transportation department, the water department, the emergency/rescue services, etc.) may be in need of similar communication services. As is well-known to those familiar with trunking theory, a relatively small number of radio repeaters can efficiently service all of these needs within a given geographic area if they are trunked (i.e., shared on an "as-needed" basis between all potential units).

This invention also is especially adapted for special mobile radio (SMR) trunked users. Here, an entrepreneur may provide a trunked radio repeater system at one or more sites within a given geographic area and then sell air time to many different independent businesses or other entities having the need to provide efficient radio communication between individual units of their particular organization. In many respects, the requirements of an SMR user are similar to those of a PST user.

In fact, the potential advantages of trunked radio repeater systems for public services is so well recognized that an organization known as the Association of Public-Safety Communications Officers, Inc. (formerly the Association of Police Communications Officers) (APCO) has developed a set of highly desirable features for such a system commonly known as the "APCO-16 Requirements." A complete listing and explanation of such requirements may be found in available publications known to those in the art.

One of the APCO-16 requirements is that any user must have voice channel access within one-half second after engaging a push-to-talk (PTT) switch. This same requirement must especially be met in emergency situations--and that implies that the system must be able to actively drop lower priority users also within a very short time frame. And, of course, the ability to quickly and efficiently drop channel assignments as soon as channel usage is terminated is also important for efficient usage of the trunked facility even in non-emergency situations.

Prior trunked radio systems have attempted to more or less "just meet" such APCO-16 requirements of timeliness. For example, published specifications of one such prior system indicates an ability to achieve channel update (in a 19 channel system) within 450 milliseconds and channel drops within 500 milliseconds. To achieve this, it utilizes 3,600 bits per second (bps) digital signalling over a dedicated digital control channel. Unfortunately, although theoretically the APCO-16 requirements of timeliness should be met by such a prior system, in reality, the APCO-16 timeliness requirements are often not met--or, are met only at the expense of suffering with the obviously adverse effects of somewhat unreliable digital control signalling (which are, at best, annoying even in non-emergency situations). Accordingly, there is considerable room for improvement.

The present invention provides substantial improvements--both in timeliness and in reliability of critical control signalling in a digitally trunked radio system of this general type. To begin with, a much higher digital signalling rate (9600 bps) is utilized. However, rather than using all of the increased signalling rate to provide a 9600/3600=2.67 improvement factor in timeliness, a large portion of the increased signalling rate capacity is utilized to improve signalling reliability. Accordingly, the increased timeliness of 19 channel updating capability, for example, is improved by a factor of approximately 1.58 (e.g., 285 milliseconds versus 450 milliseconds) while the rest of the increased signalling capacity is utilized to increase the reliability of control signalling. At the same time, virtually all of the increased signalling capacity is utilized to improve the timeliness of channel drop ability (e.g., 190 milliseconds versus 500 milliseconds).

As previously demonstrated by Bell System Technical Journal articles on the AMPS system (e.g. "Voice and Data Transmission", by Arredondo et al, The Bell System Technical Journal, Vol. 58, No. 1, January 1979, pp 97-122), digital data rates on radio channels should be either very low (e.g., 200 hertz) or as high as the channel bandwidth permits. The present invention utilizes the maximum high speed data rates (e.g., 9600 bps on the typical 25 KHz bandwidth radio channel) for critical control channel signalling and control signalling on the working channels both immediately before and immediately after the user communication interval. In addition, sub-audible low-speed digital data is also utilized on the working channel during user communications so as to assure additional signalling reliability--and to also permit implementation of additional features.

In the exemplary embodiment, all channels (the control as well as working channels) are fully duplexed so that there may be simultaneous in-bound and out-bound signalling on all channels. In general, this invention achieves reliable and prompt communication within a trunked radio repeater system having a digital control channel and plural working channels, which working channels are assigned for temporary use by individual radio units as specified by digital control signals on the control channel.

Channel assignment is initially requested by a calling radio unit passing digital request signals to a control site over the active control channel. In accordance with channel availability, a controller at the central site assigns a specific then-available working channel to the requested communication and passes digital assignment signals out-bound over the control channel. Both the calling radio unit and the called unit(s) detect the working channel assignment and switch their transmitter and receiver operations over to the proper working channel. Thereafter, digital handshake signals are again exchanged between the control site and at least one of the radio units (e.g., the calling unit) over the assigned working channel. In response to a successful handshake on the assigned working channel, the central site then transmits digital release signals over the assigned working channel so as to release the appropriate units for communication thereover.

As one technique for increasing reliability, the initial request signals may include three-fold data redundancy (at least for critical data) while the channel assignment signals subsequently transmitted over the control channel may include as much as six fold redundancy of data (e.g., at least of critical data such as that representing the called party and the assigned channel). The handshake signals subsequently exchanged on the assigned working channel may also include three-fold data redundancy of at least critical data. In this manner, some of the increased signalling capacity made available by the high-speed data rate (e.g., 9600 bps) is sacrificed in favor of more reliable channel allocation and communication functions --while still comfortably exceeding all APCO-16 requirements.

To insure responsiveness to higher priority calls, sub-audible digital channel assignment update messages are also transmitted over the assigned working channel. These are monitored in each unit then residing on the working channels. Accordingly, if a higher priority call is directed to some unit already engaged in a communication, that unit is enabled to promptly switch operations to a new assigned working channel so as to immediately receive the higher priority call.

In addition, to accommodate late entry of called parties to an ongoing communication, digital channel assignment "late entry" messages continue to be transmitted on the control channel even after a successful channel assignment process has been effected so that late entrants (e.g., those just turning on their radio, just passing out of a tunnel or from behind a building or otherwise back into radio communication after temporary interruption, completion of a higher or equal priority call, etc.) may nevertheless be switched onto the proper assigned working channel as soon as possible. (The late entry feature, per se, is related to copending commonly assigned application Ser. No. 725,682 filed 22 Apr. 1985.)

To effect prompt and reliable termination of channel assignments, when the PTT switch of a calling unit is released, it sends a digital unkeyed message on the assigned working channel and, in response to reception of this unkeyed message at the control site, a digital drop signal is transmitted over the assigned working channel so as to immediately drop all units therefrom and thus free that working channel for reassignment. (As will be appreciated, a given radio unit will automatically revert to monitoring the control channel upon dropping from an assigned working channel.)

The system of this invention is sometimes termed a "digitally" trunked system because trunking control is effected by digital signals passed over a continuously dedicated "control" data channel. All units are programmed so as to automatically revert to the predetermined primary control channel upon being turned on or being reset. If the expected control channel data format is not there discovered, then alternate possible control channels are successively monitored in a predetermined sequence until an active control channel is discovered. In this manner, the usual control channel apparatus at the central control site may be temporarily removed from service (e.g., for maintenance purposes). This same feature also permits continued trunked system operation in the event that the regular control channel unexpectedly malfunctions or is otherwise taken out of service.

The exemplary embodiment of this invention is designed in such a way that it meets and, in many areas, surpasses all of the existing APCO-16 requirements. It may also support available voice encryption techniques, mobile digital data terminals (digital data may be passed in lieu of analog voice data during a trunked radio communication session) and/or available automatic vehicular location systems. Preferably, a fault tolerant architecture is used [see related copending commonly assigned application Ser. No. 07/057,046 GE docket 45-MR-541, filed concurrently herewith ] so as to maintain trunked system operation even if the central processor happens to fail at the control site. If digital data is to be communicated between radio units and/or the central site, then it may be processed through the system in a manner similar to analog voice signals. In particular, such digital data communication will be carried out at a rate accommodated within the existing audio pass band and will be trunked just like desired voice communications (i.e., no dedicated digital data communication channels are required). To help increase reliability of digital data communications, data transmissions (and analog voice transmissions, as well) may be voted in voting systems employing satellite receivers connected to the central control site.

In the exemplary embodiment, digital control signalling messages of the following types are utilized:

    ______________________________________                                         Channel                                                                        Type   Direction               Rate                                            ______________________________________                                         Control                                                                               INBOUND                 9600 pbs                                        Channel                                                                               Group Call                                                                     Special Call                                                                   Status                                                                         Status Request/Page                                                            Emergency Alert/Group Call                                                     Individual Call                                                                Cancel Dynamic Regroup                                                         Dynamic Regroup -- Forced Select                                               Dynamic Regroup -- Option Deselect                                             Dynamic Regroup -- Option Select                                               Login/Dynamic Regroup Acknowledge                                              Logical ID Request                                                             Programming Request                                                            OUTBOUND                9600 bps                                               Channel Assignment                                                             Channel Update                                                                 Enable/Disable Unit                                                            Dynamic Regroup                                                                Preconfiguration                                                               Alias I D                                                                      Unit Keyed/Unkeyed                                                             Emergency Channel Assignment                                                   Cancel Dynamic Regroup                                                         Dynamic Regroup -- Forced Select                                               Dynamic Regroup -- Optional Select                                             Dynamic Regroup -- Optional Select                                             Assign Group ID An Alias ID                                                    Assign Logical ID An Alias ID                                                  Status Acknowledge/Page                                                        Time Mark                                                                      Emergency Channel Update                                                       Site ID                                                                        System Operations Made                                                         Site Status                                                                    Logical ID Assignment                                                          Programming Channel Assignment                                          Working                                                                               INBOUND                 9600 bps                                        Channel                                                                               Initial Handshake                                                              Special Call Signalling                                                        Unit ID-PTT and Reverse PTT                                                    Miscellaneous                                                                  OUTBOUND                9600 bps                                               Initial handshake                                                              Channel Drop                                                                   Status Messages                                                                INBOUND                 Low Speed                                              Confirm Unit PTT                                                               OUTBOUND                Low Speed                                              Priority Scan                                                                  Falsing Prevention                                                      ______________________________________                                    

Some of the general features of the exemplary embodiment and expected benefits are summarized below:

    ______________________________________                                         FEATURE              BENEFIT                                                   ______________________________________                                         VERY SHORT AVERAGE   Practically instantaneous                                 CHANNEL ACCESS TIME  access doesn't cut off                                                         syllables.                                                Normal Signal Strength                                                                              Provides operation                                        Areas: 280 Milliseconds                                                                             which is faster than most                                 Weak Signal Areas (12 dB                                                                            coded squelch systems.                                    Sinad: 500 Milliseconds                                                        LATE ENTRY           Minimizes missed                                          Should a mobile turn on                                                                             conversations, keeps,                                     during the period in which                                                                          police up to the minute,                                  its group is involved in                                                                            minimizes call backs.                                     a conversation, the mobile                                                     will automatically be                                                          directed to join that                                                          conversation.                                                                  AUTOMATIC CHANNEL    Frequency coordination                                    SWITCHING            of the fleets and                                         Mobiles, portables, control                                                                         subfleets requires                                        stations, and consoles                                                                              no action on the part of                                  automatically switch to                                                                             any field personnel.                                      the appropriate channel.                                                       HIGH SPEED CALL PROCESSING                                                                          Dedicated control                                         Processor assigns unit                                                                              channel provides more                                     initiating the call, and                                                                            rapid channel assign-                                     all called units, to an                                                                             ments on larger systems.                                  appropriate working channel.                                                                        System size does not                                      Initial channel assignment                                                                          impact upon channel                                       communication between site                                                                          acquisition time. Control                                 controller and radio units                                                                          channel is available for                                  occurs on the control                                                                               additional functions such                                 channel.             as status and unit ID.                                    CALL RETRY           Eliminates the need for                                   Calling unit will    repetitive PTT                                            automatically repeat its                                                                            operations by the                                         request up to eight times                                                                           operator in weak signal                                   if no response is received.                                                                         situations. Terminating                                   Retries terminated upon                                                                             retries also shortens the                                 system response.     signalling time.                                          UNIT DISABLE         In hostage situations,                                    Trunked units can be these units can be                                        disabled on an individual                                                                           assigned to a special                                     basis. These disabled units                                                                         group for com-                                            continue to monitor the                                                                             munication with the                                       control channel and can be                                                                          criminals. Also, with                                     polled to determine their                                                                           automatic vehicular                                       status.              location, these units                                                          could be tracked for                                                           apprehension.                                             SUPPLEMENTARY CONTROL                                                                               Provides user and                                         CHANNEL FUNTIONALITY system operator with                                      In addition to providing                                                                            system manager features                                   channel assignments, the                                                                            not available on other                                    control channel is used                                                                             types of systems.                                         for: status messages,                                                          polling, system status,                                                        logging, late entry,                                                           dynamic regrouping,                                                            system testing and other                                                       system functions.                                                              GROUP PRIVACY        Each group has the same                                   Each group hears only his                                                                           privacy as having their                                   own group, unless    own channel with the                                      specifically programmed                                                                             additional benefits of                                    otherwise. Dispatcher                                                                               being regrouped with                                      can override for individual                                                                         other complementing                                       units or groups of mobiles                                                                          functions for emergency                                   at any time.         operations.                                               CALL QUEUING         Maintains orderly entry                                   When all channels are                                                                               procedure for busy                                        busy, call request will                                                                             system. Call requests                                     be queued until a channel                                                                           are accepted in the                                       becomes available. Unit                                                                             order they are received                                   requesting a channel will                                                                           except higher priority                                    be notified to prevent                                                                              users go to the head of                                   call backs. Members of                                                                              the line.                                                 groups already in the queue                                                    will not be reentered in                                                       the queue.                                                                     DATA COMMUNICATIONS  This feature greatly                                      System has the optional                                                                             enhances the value of                                     capability of using 9600                                                                            the system because it                                     bps data on the working                                                                             avoids the expense of                                     channels. Data       additional RF channels.                                   communications will take                                                       place on any equipped                                                          working channel and they                                                       are trunked, just as                                                           voice communication.                                                           VOICE ENCRYPTION     Voice encryption offers                                   System has the optional                                                                             the same encrypted                                        capability of using  range as for clear voice                                  available 9600 baud voice                                                                           transmissions. Voice can                                  encryption. (See, e.g.                                                                              be passed from each site                                  commonly assigned U.S.                                                                              through conventional                                      Pat. No. 4,622,680   voice grade phone lines                                   and U.S. Pat. application                                                                           or microwave links.                                       Ser. Nos. 661,597 filed 17                                                                          Only minor modifi-                                        October 1984, 661,733                                                                               cations are needed to                                     filed 17 October 1984                                                                               the base station interface                                and 661,740 filed 17 equipment to                                              October 1984.)       accommodate such                                                               sophisticated voice                                                            security systems.                                                              Any mobile can be                                                              upgraded to this system                                                        capability by adding an                                                        external module. No                                                            internal changes to the                                                        radio are required.                                       UNIT IDENTIFICATION  Each unit on each                                         All units are automatically                                                                         transmission is                                           identified when they identified by the same                                    transmit. This is true                                                                              ID regardless of the                                      regardless of whether the                                                                           agency, fleet or                                          transmission takes place                                                                            subfleet in which he is                                   on the control channel                                                                              currently operating.                                      or on a working channel.                                                                            4095 is more than twice                                   The system is able to                                                                               the logical number                                        accommodate 4095 discreet                                                                           of users in a fully                                       addresses independent of                                                                            loaded twenty channel                                     fleet and subfleet.  system so there is more                                                        than sufficient capacity.                                 TRAFFIC LOGGING      Statistics on system                                      All system information is                                                                           usage, such as peak                                       logged. Each unit trans-                                                                            loading, individual and                                   mission causes the units                                                                            group usage versus time,                                  ID, agency, fleet, sub-                                                                             as well as many other                                     fleet, channel, time, site                                                                          system parameters, are                                    and list of sites involved                                                                          available for tabulation                                  to be logged for management                                                                         and analysis.                                             reports.                                                                       TELEPHONE INTERCONNECT                                                                              All mobiles and                                           Authorized mobiles have                                                                             portables in the system                                   the ability to place and                                                                            can be interconnected                                     receive calls and patch                                                                             to the telephone system.                                  them to individuals or                                                                              Those mobiles not                                         groups of mobiles which                                                                             specifically equipped for                                 may or may not be equipped                                                                          this can be patched by                                    for telephone interconnect.                                                                         their dispatcher to                                                            maintain adequate                                                              control of system                                                              loading factors.                                          GROUP PRIORITY ASSIGNMENT                                                                           System manager can set                                    Eight priority levels are                                                                           individual group                                          provided in the system.                                                                             priorities according to                                   Each group (as well as                                                                              the criticality of their                                  each individual) is  service.                                                  assigned a priority. The flow of traffic is                                                         more easily maintained                                                         by providing recent                                                            users priority over                                                            nonrecent users of the                                                         same priority level.                                      ______________________________________                                    

In the exemplary system, 11 bits are available to determine the address of a unit within an agency, fleet or subfleet. Twelve bits are available to determine the individual identity of a particular unit (i.e., its "logical ID"). The use of 11 bits for determining group addresses within an agency, fleet or subfleet provides great flexibility such that each agency may have many different fleet and subfleet structures. Furthermore, unit identification is not limited by a particular fleet and subfleet structure. The 4,096 unit identification codes can be divided among the subfleets in a manner that best suits a particular system.

Some features of this exemplary system which are believed to be particularly unique and advantageous are summarized briefly below (order of appearance not reflecting any order of importance--nor is this list to be considered in any way exhaustive or limiting):

a) Widening the Retry Window

If a requested working channel assignment is not achieved, the request is automatically retried --and the time window in which such a retry is attempted is increased in duration as a function of the number of prior unsuccessful retries. This significantly decreases the average channel access time where noise is the real problem rather than request collisions--while still providing a recovery mechanism for request collision problems as well.

b) Better Use of Subaudible Signalling

Rather than using subaudible signalling only to confirm channel assignments, a simple counter field is employed to greatly simplify such validity checking functions and to thus free the majority of the subaudible signalling capacity for other uses--e.g., a priority scan. In the exemplary embodiment a two bit subaudible "count" field for a given channel is incremented upon each new working assignment of that channel. Thus, if a radio unit observes a change in this field, it is programmed to immediately drop back to the control channel.

c) Minimizing Priority Communique Fragmentations by Dynamically Altering Scan Functions

After initiating a priority call, a radio temporarily (e.g., for two seconds) disables the usual multiple group scan on the control channel --in favor of looking for the highly probable returned higher priority call. This reduces the possibility of getting diverted momentarily into an ongoing lower priority communique--and also perhaps missing a fragment of the next higher priority communique. A similar temporary (e.g., two seconds) scan preference (except for priority calls) for a just-previously involved call group also helps prevent fragmentation of non-priority communiques.

d) Use of Transmission-Trunked Bit in Channel Assignment

The trunking system has two trunking modes:

a) Transmission Trunked Mode in which the working channel is de-allocated as soon as the calling unit unkeys, and

b) Message Trunked Mode in which the working channel is de-allocated "n" seconds following a unit's unkeying, unless another unit keys onto the channel within such "n" seconds. "n" is called the "hang time".

By dynamically insuring that both called and calling units "know" that a transmission-trunking mode is in effect, the calling unit may immediately revert to the control channel upon PTT release--thus immediately freeing the working channel for drop channel signalling from the control site. The called units can also be positively prevented from ever transmitting on the working channel--thus avoiding multiple keying of radio units on the working channel.

e) Automatic Addressing of immediately Returned Calls

Both the called and calling units/groups are identified in the initial channel assignment signalling. The called unit captures the calling unit ID and is enabled to automatically address a return call to the just calling radio if the PTT switch is depressed within a predetermined period (e.g., 5 seconds ) after the just completed communique even if the system is in the Transmission Trunked Mode. Not only does this simplify the necessary call back procedures and minimize access times, by allowing greater application of the Transmission Trunked Mode it also increases the probability of successful message exchanges--especially in poor signalling areas.

f) 9600 bps Permits "Loose" Synchronization

Use of higher rate 9600 bps signalling permits simplified bit synchronization to be rapidly achieved by simple "dotting" sequences (i.e., a string of alternating ones and zeros 101010 . . . ). Thus, there is no need to keep information transfers precisely synchronized across all channels. This not only reduces hardware requirements system-wide, it also facilitates a more fault tolerant architecture at the control site.

g) Improved Channel Drop Signalling

The drop channel signalling is simply an extended dotting sequence. Therefore, each radio may easily simultaneously look for drop channel signalling and channel assignment confirmations. This means that the control site may more immediately consider a given working channel available for reassignment--and, if "loaded up," immediately interrupt the drop-channel signalling to issue fresh channel assignment confirmation signals on the working channel (which each individual radio will ignore unless properly addressed to it). As a result, a "loaded" system (i.e., one where existing channel requests are already queued) may drop a working channel within about only 100 msec--and immediately reassign it to a queued request. Radios that happen to enter late into the call being dropped can detect that fact and properly drop from the channel because of the ability to simultaneously look for drop channel signalling and channel assignment confirmation signalling.

h ) Feature Programming

To avoid cumbersome feature programing (and reprogramming to add features) by factory or distribution personnel, novel procedures are employed which safely permit the end user to perform all such "programing." All units are programmed at the factory to perform all available functions. A function enable bit map and a unique physical ID are together encrypted at the factory and provided to the user as "Program Codes." When the user programs each device, its encrypted "Program Codes" are input to a Radio Programmer which, in turn, properly sets the feature enable bit map in a connected radio unit--and the decoded physical ID--and a "Just Programmed" bit). The "just programmed" radio device logs into the central controller with a request for a logical ID--based on its apparent physical ID. If illegal copying of function enabling Program Codes occurs, then the same logical ID will be assigned--and the usefulness of the radio within the trunked repeater system will be diminished.

i) Double Channel Assignment Handshake--One Being on the Assigned Working Channel

A first 9600 bps channel assignment signalling exchange occurs on the control channel. However, a confirmation (i.e., a second handshake) then occurs on the assigned working channel. Thus, it is assured that the desired channel has been successfully assigned and locked onto before the central controller unmutes the called units on the assigned channel. The signalling is such that if the channel conditions are unsuitable for voice, the handshake will fail, thus terminating the call automatically.

BRIEF DESCRIPTION OF THE DRAWINGS

These as well as other objects and advantages of this invention will be more completely understood and appreciated by carefully studying the following detailed description of the presently preferred exemplary embodiment taken in conjunction with the accompanying drawings, of which:

FIG. 1 is a general explanatory diagram of trunked radio repeater system in accordance with this invention;

FIG. 2 is a simplified block diagram of a central control site (as well as a satellite receiver site) in the trunked repeater system-of FIG. 1;

FIG. 3 is a simplified block diagram of the general site architecture of the main controller for the central control site;

FIG. 4 is a simplified block diagram of the channel architecture used within each channel of the central site architecture shown in FIG. 3;

FIG. 5 is a simplified general block diagram of a technical mobile/portable radio unit to be utilized for communication within the trunked repeater system of FIG. 1;

FIG. 6 is a simplified flowchart of typical call processing sequence in the exemplary embodiment from the perspective of a calling unit;

FIG. 7 is a simplified flowchart of a call processing sequence within a called unit;

FIG. 8 illustrates the trunked channel dropping procedure and typical required drop times;

FIG. 9 generally illustrates call initiation signalling within the exemplary system as well as typical timing requirements;

FIGS. 10A and 10B generally illustrate of the control signalling protocol utilized to initiate and terminate trunked radio communications on either an individual or group basis in the exemplary system; and

FIGS. 11A and 11B are flowchart of suitable computer programs which might be utilized at the site controller, the calling unit and the called unit so as to achieve the signalling protocol of FIG. 10.

DETAILED DESCRIPTION OF PRESENTLY PREFERRED EXEMPLARY EMBODIMENTS

An exemplary trunked radio repeater system in accordance with this invention is generally depicted at FIG. 1. As illustrated, individual units of various groups communicate with each other (both within and possibly outside of their own group) via shared radio repeater channels located at a trunked repeater control site 100. The dispatch console 102 may be housed directly at the repeater station site 104 or may be remotely located via other communication facilities as will be appreciated by those in the art. There also may be multiple dispatch consoles 102 (e.g., one for each separate fleet) and a master or supervisory dispatch console for the entire system as will also be appreciated by those in the art.

The central site is depicted in somewhat more detail at FIG. 2 in conjunction with one or more satellite receiver sites 100-1. As will be appreciated, the satellite receiver sites are displaced spatially from the central site 100 such that radio reception may temporarily be better at one or the other of the chosen antenna sites. Thus, received signals from the satellite sites as well as the central sites are combined in "voter" circuitry so as to choose the best available signal for control or communication processes.

At the central site, a transmitting antenna 200 and receiving antenna 202 (which may sometimes be a common antenna structure) may be utilized with conventional signal combining/decombining circuits 204, 206 as will be apparent to those in the art. The transmitting and receiving RF antenna circuitry 200-206 thus individually services a plurality of duplex RF channel transmit/receive circuits included in a plurality of RF repeater "stations" 300, 302, 304, 306, etc. Typically, there may be 20 such stations. Each station transmitter and receiver circuitry is typically controlled by a dedicated control shelf CS (e.g., a microprocessor-based control circuit) as is also generally depicted in FIG. 2. Such control shelf logic circuits associated with each station are, in turn, controlled by trunking cards TC (e.g., further microprocessor-based logic control circuits) 400, 402, 404 and 406. All of the trunking cards 400-406 communicate with one another and/or with a primary site controller 410 via control data bus 412. The primary site controller (and optional backup controllers if desired) may be a commercially available general purpose processor (e.g., a PDP 11/73 processor with 18 MHz-J11 chip set). Although the major "intelligence" and control capability for the entire system resides within controller 410, alternate backup or "fail soft" control functions may also be incorporated within the trunking cards 400-406 so as to provide continued trunked repeater service even in the event that controller 410 malfunctions or is otherwise taken out of service. (More detail on such fail soft features may be found in commonly assigned concurrently filed application Ser. No. 07/056,046 (GE docket 45-MR-541) entitled "Fail Soft Architecture For Public Trunking System".)

An optional telephone interconnect 414 may also be provided to the public switched telephone network. Typically, a system manager terminal, printer, etc., 416 is also provided for overall system management and control (together with one or more dispatcher consoles 102). A special test and alarming facility 418 may also be provided if desired.

The signal "voter" circuits 502, 504, 506 and 508 are connected so as to receive a plurality of input digital or analog signals and to selectively output therefrom the strongest and/or otherwise most reliable one of the input signals. Thus, received signals from the central site 100 are input to respective ones of the channel voter circuits 502-508 while additional similar input signals are generated from receivers in the satellite receiver site 100-1 and also input to the appropriate respective voter circuits. The results of the voting process are then passed back to the trunking card circuits 400-406 where they are further processed as the valid "received" signals.

A slightly more detailed view of the site architecture for control data communication is shown in FIG. 3. Here, the PDP 11/73 controller 410 is seen to communicate over a 19.2 Kilobit link 412 with up to 25 trunking control cards TC controlling respective duplex repeater circuits in those individual channels. Another high-speed 19.2 Kilobit link 420 is used to communicate with the hardware that supports the down link to/from the dispatch console 102. Other data communication with the central processor 410 is via 9600 baud links as shown in FIG. 3. The central processor 410 may include, for example, a 128 Kilobyte code PROM, 1 Megabyte of RAM and 32 DHV-11/J compatible RS-232C ports. It may typically be programmed using Micropower Pascal to provide a multi-tasking, event-driven operating system to manage all of the various data communication ports on an acceptable real time basis.

At each controlled repeater channel, the 19.2 Kilobit data bus 412 (as well as that from an optional back-up controller if desired) is monitored by an 8031 processor in the TC module. The TC trunking control module exercises control over the control shelf CS of its associated repeater with audio, signalling, and control busses as depicted in FIG. 4, and may typically also receive hard-wired inputs providing clock synchronization and a "fail soft" indication (e.g., indicating that normal control by the central controller 410 is not available and that an alternate distributed control algorithm should then be implemented within each of the trunking control modules TC).

The general architecture of a suitable mobile/portable radio unit for use within the exemplary system is also microprocessor based as depicted in FIG. 5. Here, microprocessor 550 is provided with suitable memory 552 and input/output circuits 554 so as to interface with the radio unit display, keypad, push-to-talk (PTT) switch as well as audio circuits 556 which provide basic analog audio outputs to the speaker and accept analog audio inputs from the microphone. Auxiliary control over a modem 558 as a digital interface (e.g., to voice encryption, vehicle location or other types of digital communication subsystems) may also be provided if desired. And, of course, the I/O circuits 554 also permit suitable programmed control over RF receiver 560 and transmitter 562 which, via conventional signal combiners 564 permit two-way fully duplexed communication over a common antenna 566 as will be appreciated by those in the art.

A detailed and indepth description of all units and sub-units of such a sophisticated system would necessarily be extremely voluminous and complex. However, since those in this art are already generally familiar with digitally controlled trunked repeater systems with suitable RF transmitter and receiver circuits, programmed general purpose computer controllers, etc., no such exorbitantly detailed description is believed necessary. Instead, it would only serve to obscure the subject matter which constitutes the invention. Accordingly, the remainder of this description will concentrate on the signalling protocol utilized to initiate and terminate calls within the system since this is believed to constitute a significant improvement (both in reliability and speed)--while still facilitating the retention of many highly desirable system features and meeting or exceeding all APCO-16 requirements.

Call placement begins by the calling unit transmitting a special digital channel request signal on the dedicated control channel to the central site. In return, the central site transmits, outbound on the control channel, a special digital channel assignment signal. The calling unit then responds by switching immediately to the assigned working channel where the central site now sends an assignment confirmation message (also in high-speed digital form). If the calling unit properly receives the confirming signals on the working channel, then it responds with an acknowledgment back to the central site on the working channel to complete the second handshake (i.e., the first one was on the control channel and now one has taken place on the working channel) before the central site releases the called units to begin the requested communication session on the working channel. Alternatively, if during this process the calling unit receives a channel update message on the control channel addressed to it, then the channel request call is temporarily suspended (unless the channel request under way is an emergency or higher priority request) and the calling unit then reverts to the called state so as to receive the incoming call. If the calling unit receives no response (or an improperly completed response handshake sequence), it automatically waits a random period before retrying to successfully place the call request (up to a maximum of 8 tries).

The called unit initially resides in a standby configuration where it continually monitors the digital messages appearing on the control channel outbound from the central site. If it detects a channel assignment message addressed to it as the called party (or perhaps as one party of a called group), then the called unit immediately switches its operations onto the assigned working channel. There, it also detects the confirmation signals outbound on the working channel from the central site and, if successfully confirmed, awaits a release or unsquelching signal on the working channel (e.g., transmitted from the central site in response to successful completion of a handshake with the calling unit on the working channel). The called unit may also receive a channel update message indicating that the group; is already operating on a working channel.

The programming for a calling unit is generally depicted in the simplified flowchart of FIG. 6. Here, upon entry into the calling mode, a call request is sent on the control channel CC at step 600. A test is made at 602 for call queuing. If queued, transfer is made to wait loop 603 (including a test for a detected assignment at step 604 followed by a check for expiration of a 30 second timer at 606 (whereupon control effectively is passed back to a manual requirement to restart the calling process via exit 607).

If the call request is not queued, then a test is made at 608 to see if this particular unit has already previously been requested as a called party. If so, then transfer is made at 610 to the called mode of operation. If not, then a check for a returning channel assignment is made at 612. If not received at the expected time, then a random wait is interposed at 614 before a test is made at 616 to see if eight tries have yet been made to complete this particular call. If so, then the subroutine is exited at 618. If not, then the subroutine is re-entered at 600.

If a channel assignment is successfully detected at either 612 or 604, then the unit operation is immediately switched to the assigned working channel at step 620 and a test for the second successful handshake (confirmed signalling) is made at 622. If unsuccessfully confirmed on the working channel, then exit will be made and the call is terminated. However, if the second handshake (e.g., the handshake on a working channel) is successfully confirmed and completed, then the calling unit transmits an elongated sequence of dotting at 624 (e.g., representing the successful second handshake) followed by a transmission of voice at 626 (or data if a digital communication session has been requested) over the assigned working channel before exit from the subroutine is taken at 628 (e.g., to a standard monitoring routine which looks for release of the PTT switch and transmits an unkeyed signal at 627).

The protocol followed by a called unit is generally depicted at FIG. 7 (e.g, representing a suitable computer program for controlling the unit in this mode of operation).

Upon entry, the control channel is simply monitored at 700 for any "good" message (e.g., one addressed to this particular unit). If such a message is detected, a check is made at 702 for an "update" type of message. If the message is of this type, then a check is made at 704 to see if it is repeated within about 1.0 seconds. If not, then reentry into the called mode is made. However, if the update of a higher priority incoming call is repeated within such period, then an immediate switch to the there-assigned working channel is made at 706. If signalling is not confirmed at 707, then an immediate switch to unsquelching (716) is made and that channel is thereafter monitored. If, on the other hand, signalling confirmation at 707 is achieved, it is an indication that normal channel assignment is, in reality, taking place and control is passed to block 714 to look for an unmute message.

If no channel update message is detected at 702, then the message is checked to see if it was a channel assignment at 708. If not, then return is made to the beginning of the subroutine. However, if a proper channel assignment has been received, then a switch to the assigned working channel is made at 710 and a check for proper confirmation signalling on the working channel is then made at 712. If a proper unmuting message is thereafter also received on that assigned working channel at 714, then the called unit unsquelches at 716. If no unmuting message is received at 714, then a check for a drop message is made at 718. If there is no drop message but high-speed signalling is still present on the working channel (as detected by 720), then a further check is made for the unmute message at 714. However, if there is no drop message and the high-speed signalling has ceased at 720, then the called unit is nevertheless unsquelched at 716.

At the conclusion of a desired audio call, the calling radio transmitter transmits a special release PTT signal as depicted graphically in FIG. 8. After a suitable transmission and detection delay period, the assigned working channel responds by transmitting a drop channel signal on the working channel. As shown in FIG. 8, this results in a typical working channel availability in only 167 milliseconds after the release PTT signal is initiated.

Typical timing of calling protocol signals is depicted graphically in FIG. 9 where in combination with Table T1 it can be seen that typical calling protocol can be completed and communication begun over the desired working channel within about 290 milliseconds.

                  TABLE 1                                                          ______________________________________                                                            MIN   TYP     MAX*                                                             (msec)                                                                               (msec)  (msec)                                        ______________________________________                                         t.sub.s = TIME TO SYNC & START TX                                                                   30      45      60                                        t.sub.tx = TX TIME   30      30      30                                        t.sub.w = WAIT TIME  30      60      60                                        t.sub.ca = CHANNEL ASSIGNMENT                                                                       30      30      30                                        TIME                                                                           t.sub.wc = TIME TO WORKING                                                                          15      20      25                                        CHANNEL                                                                        t.sub.c = TIME TO CONFIRMATION                                                                      25      25      60                                        t.sub.cd = TIME BETWEEN CON-                                                                        20      20      25                                        FIRMATION & DOTTING                                                            t.sub.d = TIME FOR SENDING                                                                          60      60      60                                        DOTTING                                                                        t.sub.u = TIME TO UNMUTE                                                                             0       0      30                                                             240     290     380                                       ______________________________________                                          *MAXIMUM TIME ASSUMING SUCCESS                                           

Some bit-level maps of some relevant message formats (and other related signalling formats and protocol) are graphically illustrated at FIGS. 10A and 10B. The control channel transmits an outbound continuous transmission repeating the format 800 depicted in FIG. 10A. As will be seen, each 40 bit message is transmitted three times (including one inverted transmission where all 0's are changed to 1's and vice versa) and there are two such messages transmitted per recurring message time "slot." As will be appreciated, the optional dotting prefix (if used) insures continued bit synchronization by receiving units and the unique Barker code permits frame synchronization so as to define bit boundaries between the 40 bit-level messages which follow. Since the control channel transmits these message slots continuously, no dotting prefix is needed and but one transmission of the word framing Barker code will suffice for each recurrent transmission cycle. Of course, if desired, a relatively short dotting prefix may be used to even further insure continued bit synchronization.

Inbound messages on the control channel CC are of the format 802 shown in FIG. 10A and may comprise, for example, group/individual channel assignment requests transmitted from a calling unit. Here, the dotting prefix is considerably longer and the word framing Barker code is repeated three times so as to insure that the receiving circuits at the central site are properly synchronized before the 40 bit messages (again with three-fold redundancy) are transmitted. Preferably, suitable transmission timing circuits are utilized so as to make such incoming control channel messages Synchronously time "slotted" --meaning that the messages on the inbound portion of the control channel occur during the same time slot as outgoing messages from the central site on the control channel (as generally indicated by dotted lines in FIGS. 10A and 10B).

A group call request message format is shown in expanded scale in FIG. 10. It includes a two-bit message type (MT) code (the message-type field may be extended in a tree-logic fashion to include additional bits as will be appreciated). This MT-A field thus distinguishes a group call from an individual call, for example. A type of communications field comprising two-bits indicates the type of communication session being requested. (If desired, a priority field of one-bit also may be used to indicate if a highest priority emergency call is being requested. ) The called identification code of 11 bits (representing either a group or an individual unit) is followed by a 12-bit field representing the identity of the calling unit ("logical ID"). The 40-bit message concludes with 12 bits of standard BCH code for error detection and correction purposes as will be appreciated.

The returning channel assignment message actually comprises a two message pair also having a format as shown in expanded scale at FIG. 10A. The first two bits identify the message type (MT) and the next two bits identify the type of communication session which is to take place. The identity of the calling unit is next represented by a six-digit field (e.g., with the 6 most significant bits being transmitted in one message of the two-message pair while the 6 lowest significant bits are transmitted in the other of such messages). The next one-bit field identifies whether a group call or an individual call is involved and the assigned working channel is identified by the following 5 bits. The group or individual identity of the called unit(s) is contained within the next 12 bits followed by 12 bits of BCH error detection/correction code.

Once operation reverts to the assigned working channel, the central site transmits a confirmation message of format 804 outbound on the working channel. As will be observed, it is of the same general form as the continuous transmissions on the control channel CC except that the message length has been reduced to 32 bits on the working channel. Once again, the message is sent with three-fold redundancy (one being inverted). Preferably, the confirmation message is timed in the working channel so as to be within the same time slot as messages being transmitted on the control channel. The format of the 32 bit confirmation message is also depicted in expanded form at 806 in FIG. 10. Here, 4 bits are devoted to the message type code while 2 additional bits provide a subaudible frame count useful in framing and otherwise decoding the lower speed subaudible digital data (which will subsequently appear on the working channel to be monitored by units residing thereat). One bit is also devoted to identifying the communication session as one which is transmission trunked or one which is message trunked. Another bit of the confirmation message 806 identifies the call as being either of a group or an individual unit while the identity of the called group or individual unit is contained within the following 12 bits. The confirmation message 806 concludes with 12 bits of BCH error detection/correction code.

Once the second handshake (i.e., on the working channel) has been successfully concluded, the calling unit transmits 384 bits of dotting followed by audio (in the case of a requested audio communication session) as is also depicted at FIGURE 10B.

The elongated dotted sequence transmitted by the calling unit on the working channel constitutes an acknowledgment of the successful handshake sequence and, in response, the central site transmits an outbound digital message on the working channel to positively unmute the called unit(s). The format 808 of such an unmute message is depicted in FIG. 10B. Once again, the message type code uses the first four bits while a subaudible frame count constitutes the next two bits. The next bit denotes trunked status or non-trunked status (e.g., regular hang-time) while the next bit is effectively unused (e.g., preset to zero in all unmute messages)--but which may be used for other optional purposes. The identity of the unit(s) to be unmuted is set forth in the next 12 bits followed by 12 bits of standard BCH error detection/correction code.

At the conclusion of a communication session on the working channel, the calling unit again transmits 384 bits of dotting followed by 4 data blocks of 128 bits each. Each such data block includes 16 dotting bits and a 16 bit Barker code (some of which bits may be "filler" as will be appreciated) as prefix followed by 8-bit bytes, each of which is transmitted with three-fold redundancy (one inverted)--thus constituting a 32-bit message characteristic of digital messages being transmitted on the working channel. The format of the 32-bit unkey message 810 is also shown in FIG. 10B. Here, a 4-bit message type code is followed by 2 unused bits and 2 bits for a block count. The identity of the calling unit is set forth in the next 12 bits followed by 12 bits of standard BCH error detection/correction code.

Finally, in response to receipt of the unkey message at the central site on the working channel, an outbound digital message on the working channel of a super-extended dotting sequence (e.g., 896 to 2816 bits) is transmitted from the central site as depicted at 812 in FIG. 10B--and in response, all units then on that channel drop from that particular working channel and revert to the active control channel.

The sequence of programmed events occurring at the site controller, the calling unit and the called unit(s) during a typical call origination/termination sequence is depicted in the parallel flowcharts of FIG. 11A and 11B.

Each programmed unit has a quiescent control channel (CC) monitor routine where, in a quiescent state, all units and the site controller reside. When the calling unit enters the calling subroutine from the CC monitor at 1100, a test is made at 1102 to see if this calling attempt is a retry. If not, the retry counter is set to a maximum content of 8 at 1106 and then decremented by one at 1108 (which step is directly taken from test 1102 if a retry is in progress). If the retry counter has been decremented to zero as tested at 1110, then a failed acquisition audible beep is generated at 1112 and exit is taken back to the CC monitor. On the other hand, if the maximum number of retries have not yet been made, then a channel assignment request is transmitted on the control channel and slot synchronization at 1114 (e.g., at time t₁).

Upon detecting an inbound message, the site controller will receive and store the channel assignment request and assign a free working channel at step 1200. In the exemplary system, a response to an inbound request may be supplied within a predetermined delay. The outbound channel assignment messages (i.e., a message pair) are transmitted on the control channel as soon as possible at step 1204 (time t₂). The two message channel assignment pair is then received and stored from the control channel in the calling unit at step 1118 (the unit will look for the messages up to the maximum number of slots). If either message of the two message pair is successfully received, this will suffice. As previously explained, if a channel update is received in the interim, then an exit may be taken to a called state (assuming that the call request under way is not an emergency). If a valid channel assignment message has nor been received as tested at 1120 and the maximum number of slots have been observed, then a suitable delay is loaded at 1122 an exit is taken back to the CC monitor (from which a return entry to the calling subroutine will soon be taken).

The process of loading in a suitable delay before retrying may be thought of as a progressive "widening" of the retry window--in a consciously controlled manner. There are three reasons why an inbound data message from a radio would not get a response: (1) the inbound message was not successfully detected; (2) the outbound message was not successfully detected; or (3) a collision occurred (two or more mobiles sent in a request on the same inbound control channel slot).

Given that a collision has occurred, unless mobiles randomly retransmit their requests, collisions will continue to occur. Consequently, when a radio fails to receive a response to an inbound message, it waits a "random" period of time to retransmit its request. However, if case (1) or (2) has occurred, there is really no reason to randomize the retry. Unfortunately, the radio cannot determine the cause of a failed response.

But, the longer a mobile waits to retransmit, the longer the average access time becomes in poor signalling areas since that is where the majority of retries take place. Since often it is noise and not collisions that cause missed responses, randomizing retries is often wasteful.

To correct this problem, the present invention takes some corrective action. First, non-channel-acquisition messages are caused to have a much slower retry rate than channel request messages. Access time for the former is not critical (whereas it is for the latter). So, if a collision occurs between a radio sending in a non-channel request message and a radio sending in a channel request message, the former's retry rate will be slow enough to guarantee no chance of a collision with the latter on the next retry.

Second, the random retry rate varies with the retry number. The retry algorithm (for channel acquisition messages only) widens the width of the retry window with each succeeding retry. This decreases the average access time in the presence of noise but still provides a recovery mechanism should the cause for a missed response be a collision.

The preferred embodiment uses the following simple rule:

    ______________________________________                                         1st retry         2 slot random variability                                    2nd retry         4 slot random variability                                    succeeding retries                                                                               8 slot random variability                                    ______________________________________                                    

It also is possible to vary the retry window width as a function of the received bit error rate in order to gain still greater efficiency.

If a valid working channel assignment has been received as tested at 1120, then the calling unit switches immediately to the assigned working channel at 1124 and waits to receive a proper confirmation message on the working channel at 1126--which confirmation message is being transmitted by the site controller at step 1206 at time t₃. If the confirmation message is overridden by a drop message as tested at 1128 or by timeout of a preset timer at 1130, then the calling routine is aborted and return is taken to the CC monitor. On the other hand, if a proper confirmation message is received at 1126, then the calling unit begins to transmit 384 bits of dotting on the working channel at 1132 followed by voice transmissions (or other desired communication session) at 1134.

Back at the site controller, a check is made at 1208 for the acknowledgment dotting of extended duration on the working channel. If it is not received, then exit is taken. However, if it is properly received, then two unit-keyed/unmute messages are transmitted outbound on the working channel at step 1210.

While all of the above has been taking place, the called unit has (if everything is working properly) received and stored the two-message channel assignment pair from the control channel at time t₂ (at substantially the same time as the calling unit) as depicted at 1300. (Once again, seeing either message of the two message pair is sufficient. ) In response, the called unit is also switched to the assigned working channel at 1302 and has thereafter monitored the assigned working channel for the proper confirmation thereon at step 1304 (and at time t₃. Only if the proper confirmation message has been received does the called unit then look for and receive the unmute message transmitted from the site controller on the working channel at time t₅ and, in response, unmutes the receiver of the called unit on the working channel at 1306.

During the ensuing communication session on the assigned working channel between the calling and called unit(s), the site controller (via the TC's) continues to send subaudible new channel assignment (and drop) data to all units on all working channels at 1212 (thus enabling higher priority calls to be promptly received and accepted by all units). The site controller (via the proper TC) also continues to transmit channel update messages periodically on the control channel at 1214 (e.g., so as to permit late entrants to immediately go to the proper working channel). The site controller informs all TC's of the channel assignments and drops and, in response, each TC generates suitable subaudible signalling for its channel.

In existing systems subaudible signalling typically is used as a validity check by mobiles. When a mobile is on a working channel it monitors the subaudible signalling to make sure it belongs on that channel. There are at least two reasons why a radio could get onto a channel where it does not belong:

1) being correctly within a communique on a working channel,it fails to see the channel drop or

2) monitoring the control channel, it incorrectly decodes a message and goes to an incorrect channel.

Problem (1) is solved by giving a two bit subaudible count to all radios on the channel. Every time a call is placed on a channel, the channel TC increments its count. Consequently, if a radio sees the count change, it "knows" it missed a channel drop sequence.

As for problem (2), there is a sufficiently high probability of incorrectly decoding outbound control messages on existing systems that a quick way to redirect radios from channels where they do not belong is typically provided. To do this, subaudible signalling typically is used exclusively for this purpose. However, with this invention, advantage is taken of the high information rate on the control channel, and a mobile is required to see an update message twice before going to a working channel. There is a negligible increase in the late entry time, but the probability of going to an incorrect channel is virtually eliminated. As a result, subaudible data can also be used for another purpose . . . e.g., a priority scan.

At the conclusion of the desired communications session, the unkeying of the PTT switch in the calling unit is detected at 1136 resulting in the sending of an unkeyed message on the working channel at 1138 (time t₆). If in a transmission-trunked mode, the calling unit may immediately revert to the control channel--thus immediately freeing the working channel In response, at 1216, the site controller receives the unkeyed message on the working channel and, at 1218, sends a super elongated dotting string (896 to 2816 bits on the working channel at time t₇). The called unit has, of course, also received the unkeyed message on the working channel at time t₆ and, in response, has already muted the receiver at 1308. The called unit receives the super-elongated dotting string outbound from the site controller on the working channel at time t₇ and, in response, reverts to the control channel at 1310.

A special priority scan sequence is used (in the preferred embodiment) to minimize communique fragmentation.

When a radio unit scans for multiple groups and a call is made to its priority group, the radio automatically disables the multiple group scan (in favor of a priority group only scan) for a two second interval upon returning to the control channel. Since the priority group was just communicating, the probability is high that another communique will take place within this interval. If the radio immediately scanned into another (non-priority) group call (which by definition is of a lower priority), and another communique then occurs on the priority group, the radio would hear a communique fragment from a non-priority group--and would have its entry into the next priority group communique delayed (priority scan typically may take between 1.0 and 1.5 seconds to get the radio into the priority group).

Another unique feature used to minimize communique fragmentation is a preference priority the radio automatically assigns to a just-previously monitored non-priority group. In essence, if a non-priority communique is monitored, for two seconds following the communique the radio will ignore all other scanned calls (except to the priority group of course) . . . similarly to priority group communiques. In addition, the radio always remembers the last non-priority group monitored. Upon returning from a priority group communique, the radio will prefer the last non-priority group monitored over any other groups being scanned.

In the example below, a `--` means the group is involved in a communique channel, and it is assumed that Group A is the priority group and that its communiques are separated by less than 2 seconds:

    ______________________________________                                         Group A                                                                        Group B                                                                        Group C                                                                        Radio  BBBAAA    AAAA    AA   AAAAA  BBBB  CCCC                                monitors:                                                                      ______________________________________                                    

There is a bit (i.e., the message/transmission trunked bit) in the working channel confirmation signalling that informs the radios as to whether the communique is transmission-trunked or message-trunked. This unique feature offers greater frequency efficiency.

The calling radio will be on the working channel and is guaranteed to see the message/transmission trunked bit. If the bit is set to "Transmission Mode," the calling mobile knows the channel will be removed as soon as it stops transmitting. Consequently, when its PTT is released, the calling radio automatically and immediately goes back to the control channel. This gains channel usage efficiency because the working channel TC can begin channel drop signalling as soon as it detects the calling mobile's unkey message. That is, one does not have to extend the signalling to make sure the transmitting mobile finishes transmitting, gets its receiver on channel and then has plenty of time to be guaranteed that the channel drop signalling is detected in the calling mobile.

Radios that are called also look at this message/transmission trunked bit, but for an entirely different reason. If the communique is message-trunked, radios that were called must be able to key on the assigned working channel in case they must offer a response before the channel drop. However, if the communique is transmission-trunked, none of the called radios should ever transmit on the assigned working channel. Therefor, if the bit is set to "Transmission" mode, called radios will not be permitted to key on the working channel. This is a very useful feature since it prevents radios from keying on top of each other.

The message/transmission trunked bit thus offers three system advantages: It makes transmission trunking more frequency (i.e., channel) efficient by decreasing the channel drop time (by a factor of three from typical prior systems), it reduces the dead time between transmission where users can not key (e.g., on typical existing systems, if a radio is keyed during the 0.5 second drop sequence it must wait until the sequence is complete), and it offers absolute protection from radios keying on top of each other on working channel.

To make an Individual Call on the exemplary system, the calling radio uses a single inbound slot of the control channel to identify itself and to specify the radio being called. Both radios are referred, via an outbound control channel message, to an available working channel where the confirmation signalling takes place. The unmute message to the called radio (at the completion of the high speed confirmation signalling) also specifies the ID of the calling radio. The called radio automatically stores the ID of the calling radio and, if the PTT switch of the called radio is depressed within 5 seconds of the last PTT release of the calling station, will automatically place an individual call back to the original calling. This capability allows easy user-convenient transmission trunking, and therefor better frequency (i. e., channel) efficiency, during individual calls. It also allows the calling radio to contact a called radio and converse with no channel hang time even though the called radio is not previously programmed to initiate a call to the calling radio.

The signalling in the exemplary embodiment is extremely efficient, minimizing the channel drop time and therefor increasing system efficiency. It is unique in that, for example, it is high speed signalling as opposed to low speed typically used on all other existing systems. In addition, the signalling is designed specifically for minimizing message traffic in a distributed architecture site.

Without such novel channel drop signalling, as the channel starts dropping, a message would have to be sent from the working channel TC to the control channel TC (via the site controller) stopping all updates on the outbound control channel (i.e., those that are referring radios to the working channel now being dropped). Once off the air, the channel TC would have to send an additional message to the site controller informing it of such so it can reassign the dropped working channel when appropriate. Besides additional messages within the central site slowing the channel drop process, such prior techniques also incur additional loading on the site controller. Another aspect of the problem is that the drop channel signalling that is transmitted on the working channel must be of sufficient duration to guarantee that timing ambiguities don't permit a radio to enter late onto the channel once it is down . . . or even worse to enter late onto the channel after the next call has already started to take place on that channel.

The exemplary embodiment uses unique drop channel signalling, a unique radio signalling detection algorithm and timing of when the channel TC sends the drop channel message to the site controller.

By making the drop channel signalling 9600 bps dotting, not only can the drop channel signalling be detected and muted in radios prior to the radio operator hearing the signalling, but the detection algorithm places a light enough processor loading on the radios that they can simultaneously look for the dotting and for confirmation signalling.

The following rules are followed by a working channel TC as it drops:

1) Transmit 100 msec of dotting.

2) Without interrupting the dotting, send a channel drop message to the site controller.

3) Transmit an additional 200 msec of dotting . . . BUT . . . stop it and start sending a confirmation message should a channel assignment message be received from the site controller.

The following rules are followed by the site controller when it receives a drop channel message from a given channel TC:

1) Immediately inform the control channel TC so it can stop transmitting updates to the working channel TC.

2) Consider the channel immediately available for reassignment.

The following rules are followed by a radio as it leaves a working channel:

1) For 1/2 second ignore all channel updates to the group and channel of the communique being left.

The following rules are followed by a radio as it arrives on a working channel:

1) Look for dotting (i. e., of sufficiently long duration to constitute a drop channel signal) and confirmation signalling simultaneously.

2) If drop-channel dotting is seen, leave the channel.

3) If confirmation is seen then leave the channel if the ID is not correct, otherwise lock onto the signalling and do not unmute until told to do so.

4) If confirmation signalling stops, or no signalling is seen on the channel, look for subaudible and unmute.

To understand the significance of the net effect of these procedures, consider two cases: (1) when the channel is not immediately reassigned and (2) when it is immediately reassigned.

It is only possible for the radio to enter late onto the drop channel signalling 100 msec following the point when the drop channel message was sent to the site controller. So if the channel is not being reassigned, a late entering radio will see the additional dotting being transmitted and will know to drop off the channel. On the other hand, if the system is loaded (e.g, call requests are queued in the site controller) the channel immediately gets assigned to the first group in the queue. A radio that attempts to enter late into the call just dropped will see the confirmation message with the group of the next call starting to take place and will know to drop off the channel.

The bottom line is that dropping a channel in a loaded system requires only 100 msec of signalling and only one message to the site controller. Radio's that happen to enter late into the call being dropped detect that fact because of the radios ability to look for the drop channel signalling and the confirmation signalling simultaneously.

For an increase in the radio price, the PST radio manufacturer may program additional of these "features" into radios. The typical prior way to do this is to burn a unique PROM or an EEPROM at the factory. One disadvantage of this approach is the expense of uniquely programming each radio before it leaves the factory--and it is inefficient to upgrade a radio should a customer subsequently desire additional features.

However, the exemplary embodiment permits one to eliminate factory programming costs. Since each radio is programmed in the field (e.g., groups, systems, etc.) by the customer, features should be programmed into the radio at that time. The problem is how to control the programming task sufficiently to make sure the customer only programs purchased features.

Every shipment of radios that goes to a customer will have a sheet of paper which lists a set of Programing Codes and Physical IDs (one pair for each radio). Each Programming Code is an encryption of a "feature enable bitmap" and the Physical ID of the radio.

When a customer programs a radio he/she must do two things. First, he/she programs the radio. To do this, he/she selects the Programming Code representative of the features he/she has purchased for the radio and enters it in the Radio Programmer. He/she then programs the radio using the Radio Programmer while the Programming Code prevents him/her from programming disabled features. Second, the user enters the radio onto the system database via the system manager. The radio's Physical ID must be specified in order to get the radio into the database.

Anytime the Radio Programmer writes data into a radio, it sets a "Just Programmed" bit inside the radio's personality. Whenever a radio is turned on, it checks that bit. If set, the radio will use its Physical ID to request a Logical ID from the site controller before allowing its user to communicate on the trunked system. The site controller will go to the system manager data base to determine the Logical ID to be assigned to the radio. Notice that if a customer tries using the same Programming ID for programming different radios he/she will end up with the same Logical ID in each radio which means unique identification capability is lost. This is the same consequence suffered if a customer copied the PROM used in existing systems.

The result is that one has the same level of protection,--while avoiding the need to program radios in the factory. Adding features to a radio involves issuing an updated Programming ID, ambiguities in programming radios are eliminated (e.g., in existing systems a radio could be programmed to do something it was not enabled to do . . . so when a customer programs it and it doesn't work he/she cannot tell whether the radio is erroneously programmed or the feature is disabled), and no special software is written in the mobiles . . . just the Radio Programmer. This last benefit is nice since fixing a software bug relative to feature enable/disable would involve changing code in just a few computers rather than for all radios in the field.

Detailed descriptions of the signalling protocols and formats involved in many different types of call origination sequences are summarized below:

I. Radio Origination, Logical ID Acquisition Sequence

A. The CC transmits a continuous stream of control messages which all inactive mobiles receive. The messages are sent two messages to a 30-msec frame in the following frame format:

Dotting=32 bits

Barker=16 bits (e.g., 11 bits Barker code plus 5 bits dotting preamble)

Message #1=40 bits

Message #1 (inverted)=40 bits

Message #1=40 bits

Message #2=40 bits

Message #2 (inverted)=40 bits

Message #2=40 bits

B. When mobile power is turned on, it receives a site ID message from the Control Channel (CC)in the following format:

MT-A=2 bits (e.g., 11)

MT-B=3 bits (e.g.,111)

MT-C=4 bits (e.g.,1110)

Delay=2 bits

Channel=5 bits

Priority=3 bits

Homesite=1 bit

Failsoft=2 bits

Site ID=6 bits

BCH code=12 bits

Delay specifies the maximum number of control channel slots before a control channel responds to an inbound transmission. Channel specifies the channel number for the active control channel. Priority prohibits mobiles with lower priority from transmitting on the inbound control channel. Home site bit specifies whether the site ID is the home (=0) or adjacent (=1) ID.

C. If desired, and if priority allows, mobile optionally transmits a login request on the control channel in synchronism with the received control channel messages. The frame form is as follows:

Dotting=152 bits

Barker Code (repeated three times)=48 bits (including filler)

Message=40 bits

Message (inverted)=40 bits

Message=40 bits

The login message is coded as follows:

MT-A=2 bits

MT-B=3 bits

Group ID=11 bits

Logical ID=12 bits

BCH Code=12 bits

If the mobile has no logical ID, it will transmit the logical ID request message The logical ID request message is coded as follows:

MT-A=2 bits

MT-B=3 bits

MT-C=3 bits

Physical ID=20 bits

BCH Code=12 bits

D. The control channel responds with a logical ID assignment message.

II. Radio Call Sequence--Radio Origination, Group Call

A. Control channel transmits a continuous stream of control messages which all inactive mobiles receive. The messages are sent two messages to a 30-msec frame that has the following format:

Dotting=32 bits

Barker=16 bits

Message #1=40 bits

Message #1 (inverted)=40 bits

Message #1=40 bits

Message #2=40 bits

Message #2 (inverted)=40 bits

Message #2=40 bits

B. Mobile that wishes to originate a group call transmits a group channel assignment request on the control channel in synchronism with the received control channel messages. The frame format is as follows:

Dotting=152 bits

Barker (repeated three times)=48 bits

Message=40 bits

Message (inverted)=40 bits

Message=40 bits

The group call request message is coded as follows:

MT-A=2 bits

Type Communications (e.g., voice, data, interconnect or voice privacy)=2 bits

Not used=1 bit

Group ID=11 bits

Logical ID=12 bits

BCH Code=12 bits

C. The control channel responds with a channel-assignment two-message pair. Coding is as follows:

MT-A Code=2 bits

Type Communications (e.g., voice)=2 bits

1/2 Logical ID=6 MSBs or LSBs

Group/Logical=1 bit

Channel=5 bits

Group ID=12 bits

BCH Code=12 bits

D. All mobiles of the called group switch to assigned working channel and receive a confirmation message. Slotted working channel messages are transmitted using the following frame:

Dotting=32 bits

Barker=16 bits

Message=32 bits

Message (inverted)=32 bits

Message=32 bits

The group call confirmation message is coded as follows:

MT Code=4 bits

Subaudible count=2 bits

Message/Transmission Trunking=1 bit

Group/Logical ID=1 bit

Group ID=12 bits

BCH Code=12 bits

E. Originating mobile receives confirmation message and transmits 384 bits of dotting, then audio.

F. Working channel receives dotting and transmits two unit-keyed/unmute messages.

MT Code=4 bits

Subaudible count=2 bits

Message/Transmission Trunking=1 bit

Filler=1 bit

Logical ID=12 bits

BCH Code=12 bits

Called mobile receives unmute message and unmutes audio.

G. Active mobiles on other working channels receive a subaudible channel assignment message.

H. Control channel transmits channel update message for late entry mobiles.

I. Transmitting mobile unkeys and sends a non-slotted unkey message. All non-slotted message formats are:

Dotting=384 bits

Data Block #3=128 bits

Data Block #2=128 bits

Data Block #1=128 bits

Date Block #0=128 bits

Data Block #3, #2, #1 and #0 are identical--except for a two bit block count--(each block is repeated four times) and each has the following format:

    ______________________________________                                                  Dotting = 16 bits                                                              Barker Code = 16 bits                                                          Byte 1 = 8 bits                                                                Byte 1 (inverted) = 8 bits                                                     Byte 1 = 8 bits                                                                Byte 2 = 8 bits                                                                Byte 2 (inverted) = 8 bits                                                     .                                                                              .                                                                              .                                                                              Byte 3 = 8 bits                                                                Byte 4 = 8 bits                                                                Byte 4 (inverted) = 8 bits                                                     Byte 4 = 8 bits                                                       ______________________________________                                    

The unkey message is coded as follows:

MT Code=4 bits

Unused=2 bits

Block Count=2 bits

Logical ID=12 bits

BCH Code=12 bits

J. Working channel transmits 896 to 2816 bits of dotting to drop all mobiles from the channel.

III. Radio Call Sequence-Radio Origination, Individual Call

A. Control transmits a continuous stream of control messages which all inactive mobiles receive. The messages are sent two messages to a 30-msec. frame that has the following format:

Dotting=32 bits

Barker=16 bits

Message #1=40 bits

Message #1 (inverted)=40 bits

Message #1=40 bits

Message #2=40 bits

Message #2 (inverted)=40 bits

Message #2=40 bits

B. Mobile that wishes to originate an individual call transmits an assignment request on the control channel in synchronism with the received control channel messages. The frame format is as follows:

Dotting=152 bits

Barker (repeated three times)=48 bits

Message=40 bits

Message (inverted)=40 bits

Message=40 bits

The individual call request message is coded as follows:

MT-A Code=2 bits

Type Communications (e.g., voice)=2 bits

Logical ID (called)=12 bits

Logical ID (caller)=12 bits

BCH Code=12 bits

C. The control channel responds with a channel-assignment two-message pair. Coding is as follows:

MT-A Code=2 bits

Type Communications (e.g., voice)=2 bits

1/2 Logical ID=6 MSBs or 6 LSBs

Group/Logical ID=1 bit

Channel=5 bits

Logical ID=12 bits

BCH Code=12 bits

D. Both calling (last logical ID) and called mobile switch to assigned working channel and receive a confirmation message. Slotted working channel messages are transmitted using the following frame:

Dotting=32 bits

Barker=16 bits

Message=32 bits

Message (inverted)=32 bits

Message=32 bits

The individual call confirmation message is coded as follows:

MT Code=4 bits

Subaudible Count=2 bits

Message/Transmission Trunking=1 bit

Group/Logical ID=1 bit

Logical ID=12 bits

BCH Code=12 bits

E. Originating mobile receives confirmation message and transmits 384 bits of dotting, then audio.

F. Working channel receives dotting and transmits two unit-keyed/unmute messages:

MT Code=4 bits

Subaudible Count=2 bits

Message/Transmission Trunking=1 bit

Unused bit=1 bit

Logical ID=12 bits

BCH Code=12 bits

Called mobile receives unmute message and unmutes audio.

G. Active mobiles on other working channels do not receive a subaudible channel assignment message.

H. Control channel transmits channel update message for late entry mobiles.

I. Transmitting mobile unkeys and sends a non-slotted unkey message. All non-slotted message formats are:

Dotting=384 bits

Data #3=128 bits

Date #2=128 bits

Data #1=128 bits

Data #0=128 bits

Data #3, #2, #1, and #0 are identical (repeated four times) and each has the following format:

    ______________________________________                                                  Dotting = 16 bits                                                              Barker = 16 bits                                                               Byte 1 = 8 bits                                                                Byte 1 (inverted) = 8 bits                                                     Byte 1 = 8 bits                                                                Byte 2 = 8 bits                                                                Byte 2 (inverted) = 8 bits                                                     .                                                                              .                                                                              .                                                                              Byte 3 = 8 bits                                                                Byte 4 = 8 bits                                                                Byte 4 (inverted) = 8 bits                                                     Byte 4 = 8 bits                                                       ______________________________________                                    

The unkey message is coded as follows:

MT Code=4 bits

Unused=2 bits

Subcount=2 bits

Logical ID=12 bits

BCH Code=12 bits

IV. Radio Call Sequence-Radio Origination, Emergency Group Call

A. Control channel transmits a continuous stream of control messages which all inactive mobiles receive. The messages are sent two messages to a 30-msec. frame that has the following format:

Dotting=32 bits

Barker=11 bits

Message #1=40 bits

Message #1 (inverted)=40 bits

Message #1=40 bits

Message #2=40 bits

Message #2 (inverted)=40 bits

Message #2=40 bits

B. Mobile that wishes to originate an emergency group call transmits an assignment request on the control channel in synchronism with the received control channel messages. The frame format is as follows:

Dotting=152 bits

Barker (repeated three times)=48 bits

Message=40 bits

Message (inverted)=40 bits

Message=40 bits

The emergency group call request message is coded as follows:

MT-A Code=2 bits

Type communications=2 bits

Status/C=1 bit

Group ID=11 bits

Logical ID=12 bits

BCH Code=12 bits

C. The control channel responds with two channel-assignment messages that are coded as follows:

MT-A Code=2 bits

Type communications=2 bits

1/2 Logical ID=6 MSBs or LSBs

Group/Logical ID=1 bit

Channel=5 bits

Group ID=12 bits

BCH Code=12 bits

D. All mobiles of the called group switch to assigned working channel and receive a confirmation message. Slotted working channel messages are transmitted using the following frame:

Dotting=32 bits

Barker=16 bits

Message=32 bits

Message (inverted)=32 bits

Message=32 bits

The emergency group call confirmation message is coded as follows:

MT Code=4 bits

Subaudible count=2 bits

Message/Transmission Trunking=1 bit

Group/Logical ID=1 bit

Group ID=12 bits

BCH=12 bits

E. Originating mobile receives confirmation message and transmits 384-bits of dotting, then audio.

F. Working channel receives dotting and transmits two unit-keyed/unmute messages.

MT Code=4 bits

Subaudible Count=2 bits

Message/Transmission Trunking=1 bit

1 bit (unused)=0

Logical ID=12 bits

BCH Code=12 bits

Called mobile receives unmute message and unmutes audio.

G. Active Mobiles on other working channels receive subaudible channel assignment message.

H. Control channel transmits channel update message for late entry mobiles.

I. Transmitting mobile unkeys and sends two non-slotted unkey messages. All non-slotted message formats are:

Dotting=384 bits

Data #3=128 bits

Data #2=128 bits

Data #1=128 bits

Data #0=128 bits

Data #3, #1, #1 and #0 are identical (repeated four times) and each has the following format:

    ______________________________________                                                  Dotting = 16 bits                                                              Barker = 16 bits                                                               Byte 1 = 8 bits                                                                Byte 1 (inverted) = 8 bits                                                     Byte 1 = 8 bits                                                                Byte 2 = 8 bits                                                                Byte 2 (inverted) = 8 bits                                                     .                                                                              .                                                                              .                                                                              Byte 3 = 8 bits                                                                Byte 4 = 8 bits                                                                Byte 4 (inverted) = 8 bits                                                     Byte 4 = 8 bits                                                       ______________________________________                                    

The unkey message is coded as follows:

MT=Code=4 bits

Subaudible Count=2 bits

Logical ID=12 bits

BCH Code=12 bits

V. Radio Call Sequence-Radio Origination, Status Call

A. Control channel transmits a continuous steam of control messages which all inactive mobiles receive. The messages are sent two messages to a 30-msec. frame that has the following format:

Dotting=32 bits

Barker=16 bits

Message #1=40 bits

Message #1 (inverted)=40 bits

Message #1=40 bits

Message #2=40 bits

Message #2 (inverted)=40 bits

Message #2=40 bits

B. Mobile that wishes to originate a status call transmits a status request on the control channel in synchronism with the received control channel messages. The frame format is as follows:

Dotting=152 bits

Barker (repeated three times)=48 bits

Message=40 bits

Message (inverted)=40 bits

Message=40 bits

The status request message is coded as follows:

MT-A Code=2 bits

MT-B Code=3 bits

MT-C Code=3 bits

3 bits (unused)=000

Auto Response=1 bit (e.g., yes )

4 bits (unused)=0000

Logical ID=12 bits

BCH Code=12 bits

C. The control channel responds with a status page message that is coded as follows:

MT-A Code=2 bits

MT-B Code=3 bits

MT-C Code=4 bits

2 bits (unused)=00

Auto response=1 bit (e.g., yes)

Status=4 bits

Logical ID=12 bits

BCH Code=12 bits

D. Called mobile transmits a control channel status message that is coded as follows:

MT-A Code=2 bits

MT-B Code=3 bits

MT-C Code=3 bits

3 bits (unused)=000

Auto Response=1 bit (e.g., yes)

Status=4 bits

Logical ID=12 bits

BCH Code=12 bits

E. Control channel responds with a status acknowledge message that is coded as follows:

MT-A Code=2 bits

MT-B Code=3 bits

MT-c Code=4 bits

2 bits (unused)=00

Auto Response=1 bit (e.g., yes)

Status=4 bits

Logical ID=12 bits

BCH Code=12 bits

Originating mobile receives status message.

VI. Radio Call Sequence-Radio Origination, Special Call

A. Control channel transmits a continuous stream of control messages which all inactive mobiles receive. The messages are sent two messages to a 30-msec. frame that has the following format:

Dotting=32 bits

Barker=16 bits

Message #1=40 bits

Message #1 (inverted)=40 bits

Message #1=40 bits

Message #2=40 bits

Message #2 (inverted)=40 bits

Message #2=40 bits

B. Mobile that wishes to originate a special call transmits a special call request on the control channel in synchronism with the received control channel messages. The frame format is as follows:

Dotting=152 bits

Barker (repeated three times)=48 bits

Message=40 bits

Message (inverted)=40 bits

Message=40 bits

The special call request message is coded as follows:

MT-A Code=2 bits

MT-B Code=3 bits

MT-C Code=3 bits

2 bits (unused)=00

Type Communications Code=2 bits (e.g., interconnect)

1 bit (unused)=0

Priority Code=3 bits

Logical ID=12 bits

BCH Code=12 bits

C. The control channel responds with a channel-assignment two-message pair. Coding is as follows:

MT-A Code=2 bits

Type Communication Code=2 bits (e.g., intent)

1/2 Logical ID=6 MSBs or LSBs

Group/Logical=1 bit

Channel=5 bits

Logical ID=12 bits

BCH Code=12 bits

D. Mobile switches to the assigned working channel and receives a confirmation message. Slotted working channel messages are transmitted using the following frame.

Dotting=32 bits

Barker=16 bits

Message=32 bits

Message (inverted)=32 bits

Message=32 bits

BCH Code=12 bits

The special call confirmation message is coded as follows:

MT Code=4 bits

Subcount=2 bits

Hang/time/Trunked=1 bit

Group/Logical ID=1 bit

Logical ID=12 bits

BCH Code=12 bits

E. Originating mobile receives confirmation message and transmits a multi-block special call message. The message frame (shown below) can have from 1 to 16 blocks.

    ______________________________________                                                  Dotting = 384 bits                                                             Data #3 Block #1 = 128 bits                                                    Data #2 Block #1 = 128 bits                                                    Data #1 Block #1 = 128 bits                                                    Data #0 Block #1 = 128 bits                                                    Data #3 Block #2 = 96 bits                                                     Data #2 Block #2 = 96 bits                                                     .                                                                              .                                                                              .                                                                     ______________________________________                                    

Data #3, #2, #1, #0 are identical in each block (repeated four times). Coding for Block #1 Data is:

    ______________________________________                                                  Dotting = 16 bits                                                              Barker = 16 bits                                                               Byte 1 = 8 bits                                                                Byte 1 = (inverted) = 8 bits                                                   Byte 1 = 8 bits                                                                Byte 2 = 8 bits                                                                Byte 2 (inverted) = 8 bits                                                     .                                                                              .                                                                              .                                                                              Byte 3 = 8 bits                                                                Byte 4 = 8 bits                                                                Byte 4 (inverted) = 8 bits                                                     Byte 4 = 8 bits                                                       ______________________________________                                    

Data in blocks after block #1 do not have dotting or Barker code. If telephone interconnect is required, block #1 data is coded as follows:

Group Count=4 bits

Individual Count=4 bits

Phone Digit Count=4 bits

Phone Digit #1=4 bits MSD

Phone digit #2=4 bits

BCH Code=12 bits

If no interconnect is required, block #1 is coded as

Group Count=4 bits

Individual Count=4 bits

Group/Logical ID=12 bits

BCH Code=12 bits

Subsequent blocks are coded with either one group ID, one logical ID, or five telephone digits as required to satisfy the block #1 counts. The telephone digits first, then the logical ID's, then the group ID's. Digit coding is one digit per nibble (null=1010). ID coding is

8 Bite=10101010 (8 bits)

Group/Logical ID=12 bits

BCH Code=12 bits

F. Working channel transmits a slotted working channel special call receive bitmap message. Slotted working channel messages are transmitted using the following frame:

Dotting=32 bits

Barker=16 bits

Message=32 bits

Message (inverted)=32 bits

Message=32 bits

BCH Code=12 bits

The special call receive bit map is coded as follows (use of similar acknowledgment bit maps is the subject of related commonly assigned copending application Ser. No. 07/056,923 filed Jun. 3, 1987 (GE docket 45-MR-496)):

    ______________________________________                                         MT Code = 4 bits                                                               Block #1 bit = 1 bit (e.g., OK)                                                Block #2 bit = 1 bit (e.g., OK)                                                Block #3 bit = 1 bit (e.g., 0 = repeat)                                        Block #4 bit = 1 bit                                                           .                                                                              .                                                                              Block #16 bit = 1                                                              BCH Code = 12 bits                                                             ______________________________________                                    

G. Originating mobile receives bitmap message and transmits a multi-block special call message. The message frame (shown below) can have from 1 to 16 blocks.

    ______________________________________                                                  Dotting = 384 bits                                                             Data #3 Block #1 = 128 bits                                                    Data #2 Block #1 = 128 bits                                                    Data #1 Block #1 = 128 bits                                                    Data #0 Block #1 = 128 bits                                                    Data #3 Block #2 = 96 bits                                                     Data #2 Block #2 = 96 bits                                                     .                                                                              .                                                                              .                                                                     ______________________________________                                    

Data #3, #2, #1, #0 are identical in each block (repeated four times). Coding for block #1 is

    ______________________________________                                                  Dotting = 16 bits                                                              Barker = 16 bits                                                               Byte 1 = 8 bits                                                                Byte 1 (inverted) = 8 bits                                                     Byte 1 = 8 bits                                                                Byte 2 = 8 bits                                                                Byte 2 (inverted) = 8 bits                                                     .                                                                              .                                                                              .                                                                              Byte 3 = 8 bits                                                                Byte 4 -- 8 bits                                                               Byte 4 (inverted) = 8 bits                                                     Byte 4 = 8 bits                                                       ______________________________________                                    

Data in blocks after block #1 do not have dotting or Barker code.

Only blocks that have a "0" in their bitmap bit are transmitted. For example in step F, block #3 in step E would be the first block retransmitted. If no bit map is received within 100 msec. after steps E or G, all blocks are retransmitted.

Steps F and G are repeated until all blocks are received correctly (BITMAP=1's).

H. Control channel transmits a continuous stream of control messages which all inactive mobiles receive. The messages are sent two messages to a 30-msec. frame that has the following format:

Dotting=32 bits

Barker=16 bits

Message #1=40 bits

Message #1 (inverted)=40 bits

Message #1=40 bits

Message #2=40 bits

Message #2 (inverted)=40 bits

Message #2=40 bits

I. The control channel sends from 0 to 16 channel-assignment two-message pairs as required for the special call. Coding for each message is as follows:

MT-A Code=2 bits

Type Communications Code=2 bits

1/2 Logical ID=6 MSBs or 6 LSBs

Group/Logical ID=1 bit

Channel=5 bits

Logical=12 bits

BCH Code=12 bits

J. All called mobiles go to the assigned working channel and unmute (same as late entry). From this point the working channel messages are the same as a group or individual call.

VII. Radio Call Sequence-Radio Origination, Dynamic-Regroup Call

A. Control channel transmits a continuous stream of control messages which all inactive mobiles receive. The messages are sent two messages to a 30-msec. frame that has the following format:

Dotting=32 bits

Barker=16 bits

Message #1=40 bits

Message #1 (inverted)=40 bits

Message #1=40 bits

Message #2=40 bits

Message #2 (inverted)=40 bits

Message #2=40 bits

B. Mobile that wishes to originate a dynamic-regroup call transmits a request on the control channel in synchronism with the received control channel messages. The frame format is as follows:

Dotting=152 bits

Barker=(repeated three times)=48 bits

Message=40 bits

Message (inverted)=40 bits

Message=40 bits

The dynamic-regroup request message is coded as follows:

MT-A Code=2 bits

MT-B Code=3 bits

Group ID=11 bits

Logical ID- 12 bits

BCH Code=12 bits

C. The control channel responds with a dynamic-regroup message that is coded as follows:

MT-A Code=2 bits

MT-B Code=3 bits

Group ID=11 bits

Logical ID=12 bits

BCH Code=12 bits

D. Mobile acknowledges the dynamic regroup with a login message that is coded as follows:

MT-A Code=2 bits

MT-B Code=3 bits

Group ID=11 bits

Logical ID=12 bits

BCH Code=12 bits

E. Mobile may request cancellation of the dynamic-regroup with a message coded as follows:

MT-A Code=2 bits

MT-B Code=3 bits

Group ID=11 bits

BCH Code=12 bits

F. Control channel responds with a cancel dynamic-regroup message that is coded

MT-A Code=2 bits

MT-B Code=3 bits

Group ID=11 bits

Logical ID=12 bits

BCH Code=12 bits

G. Mobile acknowledges the dynamic-regroup cancellation with a login message that is coded

MT-A Code=2 bits

MT-B Code=3 bits

Group ID=11 bits

Logical ID=12 bits

BCH Code=12 bits

VIII. Radio Call Sequence-Console Origination, Group Call

A. Control channel transmits a continuous stream of control messages which all inactive mobiles receive. The messages are sent two messages to a 30-msec. frame that has the following format:

Dotting=32 bits

Barker=16 bits

Message #1=40 bits

Message #1 (inverted)=40 bits

Message #1=40 bits

Message #2=40 bits

Message #2 (inverted)=40 bits

Message #2=40 bits

B. Console that wishes to originate a group call transmits a group call message to the downlink. The group call message is coded as follows:

MID=1 byte (#0)=8 bits

#bytes=1 byte (#1)=8 bits

Source destination=bytes #2 and #3 =16 bits

Not Used=4 bits

MT-A Code=2 bits

Type communication=2 bits

Group ID=12 bits

Logical ID=12 bits

Parity=1 byte (#8)=8 bits

C. The control channel responds with a channel-assignment two-message pair. Coding is as follows:

MT-A Code=2 bits

Type Communications=2 bits

1/2 Logical ID=6 MSBs or LSBs

Group/Logical ID=1 bit

Channel=5 bits

Group ID=12 bits

BCH Code=12 bits

D. All mobiles of the called group switch to assigned working channel and receive a confirmation message. Slotted working channel messages are transmitted using the following frame:

Dotting=32 bits

Barker=16 bits

Message=32 bits

Message (inverted)=32 bits

Message=32 bits

The group call confirmation message is coded as follows:

MT Code=4 bits

Subaudible Count=2 bits

Hang Time/Trunked=1 bit

Group/Logical ID=1 bit

Group ID=12 bits

BCH Code=12 bits

E. Originating console receives channel-assignment from the downlink and switches audio to the specified channel. Console message is coded as follows:

MID=1 Byte (#0)=8 bits

# Bytes=1 Byte (#1)=8 bits

S/D=Bytes #2, #3=16 bits

Not Used=4 bits

MT Code=2 bits

Type communications=2 bits

Logical ID--12 bits

Not Used=2 bits

GR/L ID=1 bit

Channel=5 bits

Group ID=12 bits

Parity=1 Byte (#9)=8 bits

F. Working channel transmits two unit-keyed/unmute messages:

MT Code=4 bits

Subaudible Count=2 bits

Hang-time/trunked=1 bit

1 bit (unused)=0

Logical ID=12 bits

BCH Code=12 bits

Called mobiles receive unmute message and unmute audio.

G. Active mobiles on other working channels receive a subaudible channel assignment message.

H. Control channel transmits channel update message for late entry mobiles.

I. Console sends unkey message that is coded as follows:

MID=1 Byte (#0)=8 bits

# Bytes=1 byte=8 bits

Source Destination=Bytes #2 & #3=16 bits

Not used=4 bits

MT Code=4 bits

Not Used=4 bits

Logical ID=12 bits

Parity=Byte #7=8 bits

J. Working channel transmits 896 to 2816 bits of dotting to drop all mobiles from the channel.

K. Console receives unkey message:

MID=Byte #0=8 bits

#Bytes=Byte #1=8 bits

Source Designation=Bytes #2 & #3=16 bits

MT-A/B/C=9 bites

Drop Cr=1 bit

Not Used=1 bit

Channel=5 bits

Logical ID=12 bits

Parity=Byte #8=8 bits

IX. Radio Call Sequence-Console Origination, Individual Call

A. Control channel transmits a continuous stream of control messages which all inactive mobiles receive. The messages are sent two messages to a 30-msec. frame that has the following format:

Dotting=32 bits

Barker=16 bits

Message #1=40 bits

Message #1 (inverted)=40 bits

Message #1=40 bits

Message #2=40 bits

Message #2 (inverted)=40 bits

Message #2=40 bits

B. Console that wishes to originate an individual call transmits an individual call message to the downlink. The individual call message is coded as follows:

MID=Byte #0=8 bits

#Bytes=Byte #1=8 bits

Source Destination=Bytes #2 & #3=16 bits

Not Used=4 bits

MT-A Code=2 bits

Type Communication=2 bits

Logical ID=12 bits

Logical ID=12 bits

Parity=Byte #8=8 bits

C. The control channel responds with a channel-assignment two-message pair. Coding is as follows:

MT-A Code=2 bits

Type Communications=2 bits

1/2 Logical ID=6 MSBs or LSBs

Group/Logical ID=1 bit

Channel=5 bits

Logical ID=12 bits

BCH Code=12 bits

D. Called mobile switches to assigned working channel and receives a confirmation message. Slotted working channel messages are transmitted using the following frame:

Dotting=32 bits

Barker=16 bits

Message=32 bits

Message inverted=32 bits

Message=32 bits

The individual call confirmation message is coded as follows:

MT Code=4 bits

Subaudible Count=2 bits

Hang Time/Trunked=1 bit

Group/Logical ID=1 bit

Logical ID=12 bits

BCH Code=12 bits

E. Originating console receives channel-assignment message from the downlink and switches audio to the specified channel. Console message is coded as follows:

MID=Byte #0 (8 bits)

#Bytes=Byte #1=8 bits

Source Destination=Bytes #2, & #3=16 bits

Not Used=4 bits

MT Code=2 bits

Type Communication Code=2 bits

Logical ID=12 bits

Not used=2 bits

GR/L ID=1 bit

Channel=5 bits

Logical ID=12 bits

Parity=Byte #9=8 bits

F. Working channel transmits two unit-keyed/unmute messages.

MT Code=4 bits

Subaudible Count=2 bits

Hang-Time/Trunked=1 bit

1 bit (unused)=0

Logical ID=12 bits

BCH Code=12 bits

Called mobile receives unmute message and unmutes audio.

G. Active Mobiles on other working channels do not receive a subaudible channel assignment message.

H. Control channel transmits channel update message for late entry mobiles.

I. Console sends unkey message that is coded as follows:

MID=Byte #0=8 bits

#Bytes=Byte #1=8 bits

Source Destination=Bytes #2 & #3=16 bits

Not Used=4 bits

MT Code=4 bites

Not Used=4 bits

Logical ID--12 bits

Parity=Byte #7=8 bits

J. Working channel transmits 896 to 2816 bits of dotting to drop all mobiles from the channel.

K. Console receives unkey message:

MID=Byte #0=8 bits

# Bytes=Byte #1=8 bits

Source Destination=Bytes #2 and #3=16 bits

MT-A/B/C=9 bites

Drop Ch=1 bit

Not Used=1 bit

Channel=5 bites

Logical ID=12 bits

Parity=Byte #8=8 bits

X. Radio Call Sequence Console Origination, Activate Patch

A. Control channel transmits a continuous stream Of control messages which all inactive mobiles receive. The messages are set two messages to a 30-msec. frame that has the following format:

Dotting=32 bits

Barker=16 bits

Message #1=40 bits

Message #1 (inverted)=40 bits

Message #1=40 bits

Message #2=40 bits

Message #2 (inverted)=40 bits

Message #2=40 bits

B. Console that wishes to establish a patch transmits a patch ID assignment message to the downlink. The patch ID assignment message is variable length depending upon the group and individual ID counts:

MID=29=Byte #0=8 bits

# Bytes=Byte #1=8 bits

Source/Destination=Bytes #2, #3=16 bits

Not Used=4 bits

Group Count=4 bits

Individual Count=4 bits

Logical ID=12 bits

Not Used=12 bits

Logical ID=12 bits

Not Used=12 bits

Logical ID=12 bits

Not Used=13 bits

Group ID=11 bits

Not Used--13 bits

Group ID=11 bits

Not used=13 bits

Patch ID=11 bits

Parity=8 bits

C. Console receives an acknowledgement of the patch request from the site controller using a special group ID code (1000 0000 0000):

MID=12=Byte #0=8 bits

# Bytes=Byte #1=8 bits

Source/Destination=Bytes #2, #3=16 bits

Not Used=4 bits

MT-A Code=11=2 bits

MT-B Code=100=3 bits

Patch ID=11 bits

Group ID=11 bits

Parity=Byte #8=8 bits

D. When console wishes to activate the patch it transmits a patch activate message to the downlink.

MID=27=Byte #0=8 bits

#Bytes=Byte #1=05=8 bits

Source/Destination=Bytes #2, & #3=16 bits

Not Used=4 bits

MT Code=4 bits (1110)

Not Used=5 bits

Patch ID=11 bits

Parity=Byte #7=8 bits

E. The control channel responds with alias ID assignment messages. Group assignment messages are coded:

MT-A Code=2 bits (11)

MT-B Code=3 bits (110)

Alias Group ID=11 bits

Not Used=1 bit

Group ID=11 bits

BCH Code=12 bits

Group alias ID messages are repeated in the control channel background mode and not acknowledged by the mobiles.

individual alias ID assignment messages are coded as follows:

MT-A Code=2 bits (11)

MT-B Code=3 bits (101)

Alias Group ID=11 bits

Logical ID=12 bits

BCH Code=12 bits

F. All mobiles receive assignment messages but only individual call mobiles acknowledge message.

MT-A Code=2 bits (11)

MT-B Code=3 bits (110)

Alias Group ID=11 bits

Logical ID=12 bits

BCH Code=12 bits

G. Console receives patch assignment/activate messages to confirm patch. One message for each assignment:

MID=12=Byte #0=8 bits

#Bytes=Byte #1=8 bits

Source/Destination=Bytes #2 & #3=16 bits

Not Used=4 bits

MT-A Code=2 bits (11)

MT-B Code=3 bits (100)

Patch ID=11 bits

Group ID=12 bits

Parity=Byte #8=8 bits

MID=13=Byte #0=8 bits

#Bytes=Byte #1=8 bits

Source/Destination=Bytes #2 & #3=16 bits

Not Used=4 bits

MT-A Code=2 bits (11)

MT-B Code=3 bits (101)

Patch ID=11 bits

Logical ID=12 bits

Parity=Byte #8=8 bits

H. Console originates a patch call by transmitting a group call message using the patch ID:

MID=24=Byte #0=8 bits

#Bytes=Byte #1=8 bits

Source/Destination=Bytes #2 & #3=16 bits

Not Used=4 bits

MT Code=2 bits (00)

Type Communication=2 bits (00)

Patch ID=12 bits

Logical ID=12 bits

Parity=Byte #8

I. Control channel transmits group call message. Subsequent steps are same as console originated group call.

While only one exemplary embodiment of this invention has been described in detail, those skilled in the art will recognize that many variations and modifications may be made in this embodiment while still retaining many of its novel features and advantages. Accordingly, all such modifications and variations are intended to be included within the scope of the appended claims. 

What is claimed is:
 1. A method of communication within a trunked radio repeater system having a full duplex digital control channel and plural working channels, which working channels are assigned for temporary use of individual radio units specified by digital control signals on the control channel comprising,requesting and receiving assignment of a specific working channel over said control channel with 9600 bits per second digital signals, automatically switching to an assigned working channel, engaging in a digital communication session over said working channel, and releasing said working channel at the end of a communication session for reassignment by said control channel.
 2. A method as in claim 1 wherein said communication session is a digital voice communication session.
 3. A method as in claim 2 further comprising encrypting said digital voice.
 4. A method as in claim 2 further comprising encrypting said digital voice and transmitting said encrypted digital voice at 9600 baud.
 5. A method as in claim 1 further comprising transmitting the identity of each unit when that unit is transmitting.
 6. A method as in claim 1 further comprising monitoring said control channel for a communique initiated by a calling unit, recording the identity of the calling unit at the called unit, and initiating a call back to said calling unit using said recorded identity.
 7. A method as in claim 1 further comprising voting the most reliable of a plurality of received input signals from a unit for processing as the received signal.
 8. A method as in claim 1 wherein said engaging in a communication session over said working channel is via a modem.
 9. A method as in claim 1 wherein said engaging in a communication session over said working channel is via a public switched telephone network.
 10. A method as in claim 1 further comprising monitoring a primary and alternate possible control channels for a control channel data format.
 11. A method as in claim 1 further comprising distributing control of the system in the absence of central control by a central controller.
 12. A trunked radio repeater system for achieving temporary communication between calling units and called units over controller assigned working channels comprising,a full duplex digital control channel, plural working channels, said control channel carrying 9600 bps digitally encoded request, working channel assignment, and acknowledgment signals, said working channels being fully duplexed and carrying digital communication signals at 9600 bps, means at said calling units and said called units for monitoring said control channel and for switching to an assigned working channel assigned over said control channel, means for subsequently transmitting digital channel assignment late entry messages on said control channel to permit the late entry of a called unit specified by said digital channel assignment, and means for specifying the identity of the calling unit and the identity of the called unit in each transmission.
 13. A trunked radio repeater system as in claim 12 wherein said working channels support analog voice communication.
 14. A trunked radio repeater system as in claim 12 further comprisingalternate control channels and means for successively monitoring said channels in a predetermined sequence until an active control channel is discovered.
 15. A trunked radio repeater system as in claim 12 wherein said working channels support encrypted voice communication, digital signaling, digital data and analog voice transmissions.
 16. A trunked radio repeater system as in claim 12 further comprising a call site controller,said site controller including means for receiving channel requests, means for making channel assignments, means for polling and means for voting.
 17. A trunked radio repeater system as in claim 12 and further comprising an interconnect to a public telephone network.
 18. A method of radio communication within a trunked radio communications system having a digital RF control channel and at least one RF working channel, which working channel is assigned for temporary use by individual radio units based on digital trunking control signals sent over the digital RF control channel, said method comprising,sending substantially 9600 bits per second digital channel request signals over said digital RF control channel, receiving substantially 9600 bits per second digital channel assignments over said digital RF control channel, automatically switching to an assigned working channel based on said received digital channel assignment signals, engaging in digital RF communications over said assigned working channel, and releasing said assigned working channel subsequent to said engaging step for reassignment.
 19. A method as in claim 18 wherein said digital RF communications over said assigned working channel comprises digital voice communications.
 20. A method as in claim 19 further comprising encrypting said digital voice.
 21. A method as in claim 20 further comprising transmitting said encrypted digital voice at 9600 baud.
 22. A method as in claim 18 further comprising transmitting the identity of each unit when that unit is transmitting.
 23. A method as in claim 18 further comprising monitoring said control channel at a unit for a communique initiated by a calling unit, recording the identity of the calling unit at the called unit, and initiating a call back to said calling unit using said recorded identity.
 24. A method as in claim 18 further comprising voting the most reliable of a plurality of received input signals from a unit for processing as the received signal.
 25. A method as in claim 18 wherein said engaging in communications over said working channel is via a modem.
 26. A method as in claim 18 wherein said engaging in communications over said working channel is at least in part via a public switched telephone network.
 27. A method as in claim 18 further comprising monitoring a primary and alternate possible control channels for a control channel data format.
 28. A method as in claim 18 further comprising distributing control of the system in the absence of central control by a central controller.
 29. In a trunked radio repeater system for achieving RF communication between calling units and called units over controller assigned working channels a method comprising,carrying digitially encoded working channel request and working channel assignment signals substantially at 9600 bps over a full duplex digital RF control channel, carrying digital voice communication signals substantially at 9600 bps over at least one working channel, monitoring said control channel at said calling units and said called units and switching to an assigned working channel based on said working channel assignment signals carried over said control channel, subsequently transmitting digital channel assignment late entry messages on said control channel to permit the late entry of a called unit specified by said digital channel assignment, and specifying the identity of the calling unit and the identity of the called unit in at least some of the transmissions.
 30. A method as in claim 29 further including supporting analog voice communication over said working channel.
 31. A method as in claim 29 further comprisingsuccessively monitoring said channels in a predetermined sequence until an active control channel is discovered.
 32. A method as in claim 29 further comprising supporting encrypted voice communication, digital signaling, digital data and analog voice transmissions over said working channel.
 33. A method as in claim 29 further comprising,receiving channel requests, making channel assignments, polling and voting at a central call controller.
 34. A method as in claim 29 further comprising interconnecting to a public telephone network. 